I had a different one for over a year in another tank and it never exceeded 3 inches. Reading up on them it generally says their max size is 6 inches. I'd be willing to increase the tank size if they grow. I was just assuming from my experience it takes them forever to grow?

Your fantail more likely stayed small because it growth was stunted by the small size of the tank. Stunted growth creates a lot of health problems on fishes, and also dramatically shortens their lifespan. Keep in mind, most goldfishes can like up to 10 years if not more!

Nope, they can get over a foot long, and they produce alot of waiste, no less than a 15g. The reason the goldfish didn't grow was cuz it was in a small tank and it couldn't grow properly, the insides grow and the outsides cant. TRUST ME! they can live up to 20 years and grow HUGE if they ave the right care Please either rehome your fish, or get a bigger tank!!!!!!!!!!!! PLEASE LISTEN!!!!

I have a fantail goldfish. He is in a 36gal. It took him less than a year to grow to 6inches. That does not include his tail, you could add on another 2inches for that. If they have room to grow, they will.